"Hibernate" mode?

I was curious if my MacBook would remain in sleep mode when the battery is removed (similar to my wife's PowerBook, LED still pulses). I pulled the battery, and my light went out. When I replaced the battery and restarted, my desktop was gray and blurry, and there was a progress bar at the bottom. It behaved much like PC's when they hibernate.

Is there any way I can cause my computer to go into this mode on command?

Thanks!